It is known that magnetic lifters are divided into three classes depending on the type of magnets employed, i.e. permanent magnets, electromagnets and electro-permanent magnets, each type of magnets having its own advantages and drawbacks.
The lifters with permanent magnets have the advantage of an almost negligible power consumption and of a produced magnetic force which is reliably constant and independent of outer supply sources. On the other hand, it is not possible to increase the magnetic force if necessary and the magnets are exceedingly bulky for lifting heavy loads. Furthermore, the load release requires the application of a considerable mechanical power in order to create an air gap between the lifter and the load large enough to reduce the magnetic force to a value smaller than the load weight. Alternatively, the magnets have to be made movable so that they can be moved away from the load, thus decreasing the magnetic attraction, or it is necessary to provide compensator coils to temporarily generate in the load a magnetic flux opposite to the magnetic flux generated by the permanent magnets, as in FR 2616006.
On the contrary, in the lifters with electromagnets it is possible to freely vary the magnetic force by simply adjusting the current flowing in the windings which generate the magnetic field. However, any breakdown, even if very short, of the power supply immediately cancels the magnetic force and thus causes the release of the load. It is therefore evident that safety systems ensuring the supply continuity are essential.
The lifters with electro-permanent magnets succeed in overcoming the main drawbacks of the two above-described types of lifters by combining fixed polarization permanent magnets with permanent magnets of the reversible type, i.e. magnets in which the polarization is easily reversed through the application of an electrical pulse. When the polarization of the magnetic masses, fixed and reversible, results in a North-South-North-South series the magnetic flux is short-circuited within the lifter thus making the latter inoperative, whereas when the polarization of the reversible magnets is in opposition, i.e. in parallel North-South-South-North, the magnetic flux splits up passing through the polar pieces into the ferromagnetic material to be moved and the lifter is operative. The reversible magnet thus generates an adjustable magnetic flux which can also direct the flux of a conventional non-reversible permanent magnet combined therewith, so as to short-circuit the two magnets when the lifter is to be deactivated or arrange them in parallel for activating the lifter.
Since just an electrical pulse but not a continuous supply is needed for reversing the reversible magnet, the safety problems affecting electromagnets are prevented. At the same time, even though permanent magnets are used, it is possible to vary the magnetic force within some limits, and the load release is easy to carry out with a minimum power consumption and without complex structures for moving the magnets.
However, the lifters with electro-permanent magnets manufactured until today have significant use restrictions as far as the temperature of the material that can be safely lifted is concerned. In fact the reversible magnets are usually made of an aluminium-nickel-cobalt alloy (Alnico) that has a Curie point of about 800° C., while the fixed polarization magnets are made of neodymium or ferrite that have a Curie point of about 310° C. and 450° C. respectively. This means that lifters with electro-permanent magnets of Alnico-neodymium operate without problems on ferromagnetic materials with temperatures not greater than 150-200° C., whereas those with magnets of Alnico-ferrite can operate on materials up to 350-400° C.
Moreover also the commutation coils that control the reversal of the polarization of the reversible magnets have their own maximum operating temperature, whereby upon achievement of even one of these three maximum temperatures (coils, fixed and reversible magnets) the lifter must be put to rest to cool down in order to ensure the integrity of the same, and the safety of the lifting and transport operations of the hot ferromagnetic products.
In practice this means that even a lifter provided with the best fixed polarization magnets of a samarium-cobalt alloy, which has a Curie point of about 770° C., must be put to rest after about two hours of operation in the handling of ferromagnetic materials at 600° C. with a 60% operating cycle (i.e. 60% of the time in contact with the hot material and 40% not). In fact after this time of operation the average temperature of the fixed SmCo magnets is about 350° C., which is also the limit temperature recommended by the manufacturers of such material, the temperature of the reversible Alnico magnets reaches 340° C. and the commutation coils have an average temperature of 180° C., which is also close to the temperature limit.
This also depends on the fact that in traditional lifters the pole pieces are fixed to the poles with the circuit surfaces perfectly in contact with each other, i.e. without air gap, to reduce magnetic circuit leakage thus minimizing the magnetic reluctance. This arrangement, however, also facilitates the transmission of heat towards the interior of the lift when it is used in the lifting and transport of steel mill products with temperatures varying between 400° C. and 650° C. As explained above, this heat transmission considerably reduces the operating time of the lifter because it leads to risky temperatures in relatively short times in its critical components namely, in chronological order, the fixed magnets, the reversible magnets and the commutation coils.